Evaluation and characterization of reliable non-hermetic conformal coatings for microelectromechanical system (MEMS) device encapsulation

The thrust of this project was to evaluate commercial conformal encapsulation candidates for low cost aerospace applications. The candidate conformal coatings evaluated in this study included silicone elastomers, epoxies, and Parylenes with bi-layer or tri-layer designs. Properties characterized in this study included mobile ion permeation and moisture ingress resistance, interfacial adhesion variation through thermal shock cycling and 85/spl deg/C/85% RH aging. Surface Insulation Resistance (SIR), Triple Track Resistance (TTR) and die shear strength were used for the corresponding electrical and physical property characterizations. Parylene F displayed excellent properties for environmental protection. Silicone elastomers displayed less resistance to the harsh environment as compared to the Parylene family (N, C, D types), but it could provide advantages for low residual stress applications. The change in adhesion strength between Parylene C and silicone elastomers after exposure to thermal shock cycling or 85/spl deg/C/85%RH aging for different time periods were conducted from die shear test in terms of the interfacial failure. SIR values of all the candidate materials after 1000 h exposure to 85/spl deg/C/85%RH, with 100 V dc for resistance measurement, range from 1/spl times/10/sup 8/-1/spl times/10/sup 9/ /spl Omega/. Leakage current values after 1000 h exposure to 85/spl deg/C/85%RH, 175 V bias, are in the range of 10/sup -9/ to 10/sup -11/ Amp. The bi- or tri-layer conformal coating combination investigated in this study showed significant promise for encapsulation of the microelectromechanical system (MEMS) devices.     

InAlN/GaN HEMTs: a first insight into technological optimization

High-electron mobility transistors (HEMTs) were fabricated from heterostructures consisting of undoped In/sub 0.2/Al/sub 0.8/N barrier and GaN channel layers grown by metal-organic vapor phase epitaxy on (0001) sapphire substrates. The polarization-induced two-dimensional electron gas (2DEG) density and mobility at the In/sub 0.2/Al/sub 0.8/N/GaN heterojunction were 2/spl times/10/sup 13/ cm/sup -2/ and 260 cm/sup 2/V/sup -1/s/sup -1/, respectively. A tradeoff was determined for the annealing temperature of Ti/Al/Ni/Au ohmic contacts in order to achieve a low contact resistance (/spl rho//sub C/=2.4/spl times/10/sup -5/ /spl Omega//spl middot/cm/sup 2/) without degradation of the channels sheet resistance. Schottky barrier heights were 0.63 and 0.84 eV for Ni- and Pt-based contacts, respectively. The obtained dc parameters of 1-/spl mu/m gate-length HEMT were 0.64 A/mm drain current at V/sub GS/=3 V and 122 mS/mm transconductance, respectively. An HEMT analytical model was used to identify the effects of various material and device parameters on the InAlN/GaN HEMT performance. It is concluded that the increase in the channel mobility is urgently needed in order to benefit from the high 2DEG density.     

GaN MSM UV photodetectors with titanium tungsten transparent electrodes

GaN metal-semiconductor-metal (MSM) ultraviolet photodetectors with titanium tungsten (TiW) transparent electrodes were fabricated and characterized. It was found that the 10-nm-thick TiW film deposited with a 300-W RF power can still provide a reasonably high transmittance of 75.1% at 300 nm, a low resistivity of 1.7/spl times/10/sup -3/ /spl Omega//spl middot/cm and an effective Schottky barrier height of 0.773 eV on u-GaN. We also achieved a peak responsivity of 0.192 A/W and a quantum efficiency of 66.4% from the GaN ultraviolet MSM photodetector with TiW electrodes. With a 3-V applied bias, it was found that minimum noise equivalent power and maximum D/sup */ of our detector were 1.987/spl times/10/sup -10/ W and 6.365/spl times/10/sup 9/ cmHz/sup 0.5/W/sup -1/, respectively.     

GaInP/GaAs collector-up tunneling-collector heterojunction bipolar transistors (C-up TC-HBTs): optimization of fabrication process and epitaxial layer structure for high-efficiency high-power amplifiers

This paper describes a novel heterojunction bipolar transistor (HBT) structure, the collector-up tunneling-collector HBT (C-up TC-HBT), that minimizes the offset voltage V/sub CE,sat/ and the knee voltage V/sub k/. In this device, a thin GaInP layer is used as a tunnel barrier at the base-collector (BC) junction to suppress hole injection into the collector, which results in small V/sub CE,sat/. Collector-up configuration is used because of the observed asymmetry of the band discontinuity between GaInP and GaAs depending on growth direction. To minimize V/sub k/, we optimized the epitaxial layer structure as well as the conditions of ion implantation into the extrinsic emitter and post-implantation annealing. The best results were obtained when a 5-nm-thick 5/spl times/10/sup 17/-cm/sup -3/-doped GaInP tunnel barrier with a 20-nm-thick undoped GaAs spacer was used at the BC junction, and when 2/spl times/10/sup 12/-cm/sup -2/ 50-keV B implantation was employed followed by 10-min annealing at 390/spl deg/C. Fabricated 40/spl times/40-/spl mu/m/sup 2/ C-up TC-HBTs showed almost zero V/sub CE,sat/ (<10 mV) and a very small V/sub k/ of 0.29 V at a collector current density of 4 kA/cm/sub 2/, which are much lower than those of a typical GaInP/GaAs HBT. The results indicate that the C-up TC-HBT's are attractive candidates for high-efficiency high power amplifiers.     

Demonstration of the first 10-kV 4H-SiC Schottky barrier diodes

This letter reports the demonstration of the first 4H-SiC Schottky barrier diode (SBD) blocking over 10 kV based on 115-/spl mu/m n-type epilayers doped to 5.6 /spl times/ 10/sup 14/ cm/sup -3/ through the use of a multistep junction termination extension. The blocking voltage substantially surpasses the former 4H-SiC SBD record of 4.9 kV. A current density of 48 A/cm/sup 2/ is achieved with a forward voltage drop of 6 V. The Schottky barrier height, ideality factor, and electron mobility for this very thick epilayer are reported. The SBD's specific-on resistance is also reported.     

Incorporation of a frequency-dependent dielectric response for the barrier material in the Josephson junction circuit model

We extend the resistively shunted Josephson (RSJ) junction circuit model originally proposed by Stewart and McCumber to incorporate a frequency-dependent dielectric response so that the influence of free carriers in the barriers can be taken into account. The methodology that we have developed uses an iterative numerical technique to calculate the current-voltage (I-V) characteristics of a Josephson junction with a barrier exhibiting both dissipation and dispersion. We give detailed results for two barrier materials with conductivities near the metal-insulator transition: a conventional semiconductor with a relatively high mobility and a strongly scattered defect solid. We show that the incorporation of the dynamic response of free carriers in the barriers of superconductor-normal-superconductor (SNS) junctions significantly influences the dc I-V characteristics for the case of material near the metal-insulator transition with high mobility. Hysteretic anomalies occur at nonzero voltages in the I-V characteristics associated with the barrier layer's plasma frequency. The resulting features, which we call critical regions, occur when the dc junction voltage is equal to /spl planck/ / 2en/spl radic//spl omega/~/sub p//sup -2/-/spl Gamma//sup 2/, where /spl omega/~/sub p/ is the barrier's plasma frequency, /spl Gamma/ is the quasi-particle scattering rate, n is an integer, and /spl planck/ is the reduced Planck's constant. We also show that our results for SNS junctions with a low-mobility barrier material are essentially identical to the predictions of the simpler RSJ model. Since the method we develope can solve the nonlinear junction equations for a barrier with an arbitrary complex conductivity, it is also capable of including other relevant processes within the barrier, including the influence of excitation from shallow defects or very soft phonon modes, as well as boundary resistances.     

A novel GaAs FET with double camel-like gate structure

Extremely high potential barrier height and gate turn-on voltage of a novel GaAs field-effect transistor with n/sup +//p/sup +//n/sup +//p/sup +//n double camel-like gate structure are demonstrated. The maximum electric field and potential barrier height of the double camel-like gate are substantially enhanced by the addition of another n/sup +//p/sup +/ layers in gate region, as compared with the conventional n/sup +//p/sup +//n single camel-like gate. For a 1/spl times/100 /spl mu/m/sup 2/ device, a potential barrier height up to 2.741 V is obtained. Experimentally, a high gate turn-on voltage up to +4.9 V is achieved because two reverse-biased junctions of the double camel-like gate absorb part of positive gate voltage. In addition, the transistor action shows a maximum saturation current of 730 mA/mm and an extrinsic transconductance of 166 mS/mm.     

Design, fabrication, and characterization of low forward drop, low leakage, 1-kV 4H-SiC JBS rectifiers

The 1-kV 4H-SiC planar junction barrier Schottky (JBS) rectifiers were designed, fabricated, and characterized. Different p+ implantation dosages and activation anneal methods were used to determine an optimum baseline process. Using the optimized process, the forward drop of our JBS rectifiers is <1.5 V while the reverse leakage current density is <1/spl times/10/sup -5/ A/cm/sup -2/. Blocking voltage>1 kV was achieved using a single-zone junction termination extension termination. It was shown experimentally that 4-/spl mu/m p-type implantation window spacing gives an optimum tradeoff between forward drop voltage and leakage current density for these rectifiers, yielding a specific on-resistance of 3 m/spl Omega//spl middot/cm/sup 2/.     

High-reflectivity Al-Pt nanostructured Ohmic contact to p-GaN

The effect of nanoscale Pt islands on the electrical characteristics of contacts to p-type gallium nitride (GaN) has been investigated to explore the feasibility for the flip-chip configuration light-emitting diodes (LEDs) using an Al-based reflector. An as-deposited Al contact to p-GaN with a net hole concentration of 3/spl times/10/sup 17/cm/sup -3/ was rectifying. However, an Al contact with nanoscale Pt islands at the interface exhibited ohmic behavior. A specific contact resistivity of 2.1/spl times/10/sup -3//spl Omega//spl middot/cm/sup 2/ and a reflectance of 84% at 460 nm were measured for the Al contact with nanoscale Pt islands. Current-voltage temperature measurements revealed a Schottky barrier height reduction from 0.80 eV for the Al contact to 0.58 eV for the Al contact with nanoscale Pt islands. The barrier height reduction may be attributed to electric field enhancement and the enhanced tunneling due to the presence of the nanoscale Pt islands. This will offer an additional silver-free option for the p-type ohmic contact in flip-chip configuration LEDs. Theory suggests that the ohmic contact characteristics may be improved further with smaller Pt islands that will enhance tunneling across the interface with the GaN and in the vicinity of the Pt-Al interface.     

Organic TFT array on a paper substrate

We demonstrate a high-performance organic thin-film transistor array fabricated on a flexible paper substrate. As a water and chemical barrier layer, 6-/spl mu/m-thick parylene has been coated on the paper substrate by using the vacuum deposition. The parylene layer protects the paper substrate from becoming damp during the wet chemical process. Using poly (3-hexylthiophene) as an active layer, a high-performance organic transistor with field effect mobility up to 0.086 cm/sup 2//V/spl middot/s and an on/off ratio of 10/sup 4/ can be achieved. Organic transistors built on a cheap paper substrate open a channel for future applications in flexible and disposable electronics with extremely low-cost.     

Room-temperature InAsSb photovoltaic detectors for mid-infrared applications

Novel noncryogenic InAsSb photovoltaic detectors grown by molecular beam epitaxy are proposed and demonstrated. The quaternary alloy In/sub 0.88/Al/sub 0.12/As/sub 0.80/Sb/sub 0.20/ is introduced as a wide bandgap barrier layer lattice matched to the GaSb substrate. The valence band edge of In/sub 0.88/Al/sub 0.12/As/sub 0.80/Sb/sub 0.20/ nearly matches with InAs/sub 0.91/Sb/sub 0.09/, leading to more efficient transport of photogenerated holes. The resulting mid-infrared photovoltaic detector exhibits a 50% cutoff wavelength of 4.31 /spl mu/m and a peak responsivity of 0.84 A/W at room temperature. High Johnson-noise-limited detectivity (D/sup */) of 2.6/spl times/10/sup 9/ cm/spl middot/Hz/sup 1/2//W at 4.0 /spl mu/m, and 4.2/spl times/10/sup 10/ cm/spl middot/Hz/sup 1/2//W at 3.7 /spl mu/m are achieved at 300 K and 230 K, respectively.     

Low-noise silicon carbide X-ray sensor with wide operating temperature range

A silicon carbide (SiC) sensor is presented with high energy resolution in X-ray spectroscopy over a wide temperature range (27-100/spl deg/C). The sensor, consisting of a Schottky barrier diode on high resistivity epitaxial SiC, is characterised by an extremely low noise due to its ultra-low reverse current density even at high operating temperature (15 pA/cm/sup 2/ at 27/spl deg/C and 0.5 nA/cm/sup 2/ at 100/spl deg/C). Equivalent noise charges as low as 17 electrons rms at 27/spl deg/C and 47 electrons rms at 100/spl deg/C have been measured, allowing X-ray spectroscopy with an energy resolution as low as 315 eV and 797 eV FWHM, respectively.     

InAs/GaAs quantum dot infrared photodetector (QDIP) with double Al/sub 0.3/Ga/sub 0.7/As blocking barriers

The ten stacked self-assembled InAs/GaAs quantum dot infrared photodetectors (QDIP) with different Al/sub 0.3/Ga/sub 0.7/As barrier widths and growth temperatures were prepared. Asymmetric current-voltage (I-V) characteristics and 2/spl sim/7.5 /spl mu/m detection window were observed. Peak responsivity of 84 mA/W at -0.4 V and peak specific detectivity of 2.5/spl times/10/sup 9/ cm-Hz/sup 1/2//W at zero bias were observed at 50 K. The characteristics of polarization insensitivity over the incident light and the high background photocurrent suggest that the self-assembled QDIP can be operated at higher temperature (/spl sim/250 K) under normal incidence condition in contrast to quantum well infrared photodetector (QWIP).     

Quantum cyclic and constacyclic codes

Based on classical quaternary constacyclic linear codes, we construct a set of quantum codes with parameters [[(4/sup m/ -1)/3, (4/sup m/ -1)/3 -2(3l + b)m, 4l + b + 2]] where m/spl ges/4, 1/spl les/b/spl les/3, and 12l + 3b < 2 /spl times/ 4/sup /spl lfloor/(m+2)/3/spl rfloor//-1, which are better than the codes in Bierbrauer and Edel (2000).     

Time-frequency localisation of Shannon wavelet packets

It is well known that the 2/spl pi/ minimally supported frequency scaling function /spl phi//sup /spl alpha//(x) satisfying /spl phi//spl circ//sup /spl alpha//(/spl omega/)=/spl chi//sub (-/spl alpha/,2/spl pi/-/spl alpha/)/(/spl omega/), 0相似文献   

Theory and factorization for a class of structurally regular biorthogonal filter banks

Regularity is a fundamental and desirable property of wavelets and perfect reconstruction filter banks (PRFBs). Among others, it dictates the smoothness of the wavelet basis and the rate of decay of the wavelet coefficients. This paper considers how regularity of a desired degree can be structurally imposed onto biorthogonal filter banks (BOFBs) so that they can be designed with exact regularity and fast convergence via unconstrained optimization. The considered design space is a useful class of M-channel causal finite-impulse response (FIR) BOFBs (having anticausal FIR inverses) that are characterized by the dyadic-based structure W(z)=I-UV/sup /spl dagger//+z/sup -1/UV/sup /spl dagger// for which U and V are M/spl times//spl gamma/ parameter matrices satisfying V/sup /spl dagger//U=I/sub /spl gamma//, 1/spl les//spl gamma//spl les/M, for any M/spl ges/2. Structural conditions for regularity are derived, where the Householder transform is found convenient. As a special case, a class of regular linear-phase BOFBs is considered by further imposing linear phase (LP) on the dyadic-based structure. In this way, an alternative and simplified parameterization of the biorthogonal linear-phase filter banks (GLBTs) is obtained, and the general theory of structural regularity is shown to simplify significantly. Regular BOFBs are designed according to the proposed theory and are evaluated using a transform-based image codec. They are found to provide better objective performance and improved perceptual quality of the decompressed images. Specifically, the blocking artifacts are reduced, and texture details are better preserved. For fingerprint images, the proposed biorthogonal transform codec outperforms the FBI scheme by 1-1.6 dB in PSNR.     

Cyclic codes over GR(4/sup m/) which are also cyclic over /spl Zopf//sub 4/

Let GR(4/sup m/) be the Galois ring of characteristic 4 and cardinality 4/sup m/, and /spl alpha/_={/spl alpha//sub 0/,/spl alpha//sub 1/,...,/spl alpha//sub m-1/} be a basis of GR(4/sup m/) over /spl Zopf//sub 4/ when we regard GR(4/sup m/) as a free /spl Zopf//sub 4/-module of rank m. Define the map d/sub /spl alpha/_/ from GR(4/sup m/)[z]/(z/sup n/-1) into /spl Zopf//sub 4/[z]/(z/sup mn/-1) by d/spl alpha/_(a(z))=/spl Sigma//sub i=0//sup m-1//spl Sigma//sub j=0//sup n-1/a/sub ij/z/sup mj+i/ where a(z)=/spl Sigma//sub j=0//sup n-1/a/sub j/z/sup j/ and a/sub j/=/spl Sigma//sub i=0//sup m-1/a/sub ij//spl alpha//sub i/, a/sub ij//spl isin//spl Zopf//sub 4/. Then, for any linear code C of length n over GR(4/sup m/), its image d/sub /spl alpha/_/(C) is a /spl Zopf//sub 4/-linear code of length mn. In this article, for n and m being odd integers, it is determined all pairs (/spl alpha/_,C) such that d/sub /spl alpha/_/(C) is /spl Zopf//sub 4/-cyclic, where /spl alpha/_ is a basis of GR(4/sup m/) over /spl Zopf//sub 4/, and C is a cyclic code of length n over GR(4/sup m/).     

The empirical distribution of rate-constrained source codes

Let X = (X/sub 1/,...) be a stationary ergodic finite-alphabet source, X/sup n/ denote its first n symbols, and Y/sup n/ be the codeword assigned to X/sup n/ by a lossy source code. The empirical kth-order joint distribution Q/spl circ//sup k/[X/sup n/,Y/sup n//spl rceil/(x/sup k/,y/sup k/) is defined as the frequency of appearances of pairs of k-strings (x/sup k/,y/sup k/) along the pair (X/sup n/,Y/sup n/). Our main interest is in the sample behavior of this (random) distribution. Letting I(Q/sup k/) denote the mutual information I(X/sup k/;Y/sup k/) when (X/sup k/,Y/sup k/)/spl sim/Q/sup k/ we show that for any (sequence of) lossy source code(s) of rate /spl les/R lim sup/sub n/spl rarr//spl infin//(1/k)I(Q/spl circ//sup k/[X/sup n/,Y/sup n//spl rfloor/) /spl les/R+(1/k)H (X/sub 1//sup k/)-H~(X) a.s. where H~(X) denotes the entropy rate of X. This is shown to imply, for a large class of sources including all independent and identically distributed (i.i.d.). sources and all sources satisfying the Shannon lower bound with equality, that for any sequence of codes which is good in the sense of asymptotically attaining a point on the rate distortion curve Q/spl circ//sup k/[X/sup n/,Y/sup n//spl rfloor//spl rArr//sup d/P(X/sup k/,Y~/sup k/) a.s. whenever P(/sub X//sup k//sub ,Y//sup k/) is the unique distribution attaining the minimum in the definition of the kth-order rate distortion function. Consequences of these results include a new proof of Kieffer's sample converse to lossy source coding, as well as performance bounds for compression-based denoisers.     

A resonant tunneling quantum-dot infrared photodetector

A novel device-resonant tunneling quantum-dot infrared photodetector-has been investigated theoretically and experimentally. In this device, the transport of dark current and photocurrent are separated by the incorporation of a double barrier resonant tunnel heterostructure with each quantum-dot layer of the device. The devices with In/sub 0.4/Ga/sub 0.6/As-GaAs quantum dots are grown by molecular beam epitaxy. We have characterized devices designed for /spl sim/6 /spl mu/m response, and the devices also exhibit a strong photoresponse peak at /spl sim/17 /spl mu/m at 300 K due to transitions from the dot excited states. The dark currents in the tunnel devices are almost two orders of magnitude smaller than those in conventional devices. Measured values of J/sub dark/ are 1.6/spl times/10/sup -8/ A/cm/sup 2/ at 80 K and 1.55 A/cm/sup 2/ at 300 K for 1-V applied bias. Measured values of peak responsivity and specific detectivity D/sup */ are 0.063 A/W and 2.4/spl times/10/sup 10/ cm/spl middot/Hz/sup 1/2//W, respectively, under a bias of 2 V, at 80 K for the 6-/spl mu/m response. For the 17-/spl mu/m response, the measured values of peak responsivity and detectivity at 300 K are 0.032 A/W and 8.6/spl times/10/sup 6/ cm/spl middot/Hz/sup 1/2//W under 1 V bias.     

A novel four-mask-processed poly-Si TFT fabricated using excimer laser crystallization of an edge-thickened a-Si active island

A four-mask-processed polycrystalline silicon thin-film transistor (poly-Si TFT) is fabricated using 50-pulse KrF excimer laser to crystallize an edge-thickened amorphous silicon (a-Si) active island without any shrinkage. This method introduces a temperature gradient in the island to enlarge grains from the edge, especially when the channel width is narrow. The grain boundaries across the width of the channel suppress the leakage current and the drain-induced barrier lowering. Moreover, the proposed poly-Si TFT with a channel length of L = 2 /spl mu/m and a channel width of W = 1.2 /spl mu/m possesses a high field-effect mobility of 260 cm/sup 2//Vs and an on/off current ratio of 2.31 /spl times/ 10/sup 8/.